Samenvatting

This investigation tries to give more insight in the relation between the Carcinus maenas, acting as predator in this part of the food chain and the Ruditapes philippinarum, being hunted by the Carcinus maenas.
The focus in this investigation has been based on the occurrence of the different sizes of the Carcinus maenas in relation to the different sizes of Ruditapes philippinarum measured in different water temperatures. Due to the increasing demand for aquaculturally produced food, fish-farmers need more answers in their efforts to optimize their productions and to obtain higher profits. To minimize losses, they want to know more on how much and due to what cause they lose most of their R. philippinarum spat due to predation by the C. maenas. The results of this investigation are based on three separate lab experiments conducted at the facility of the HZ University of Applied Sciences.
The main research question being investigated and being expanded on in this report is: What is the relationship between the size of R. philippinarum spat predated by the different size of C. maenas in different temperatures in the Zeeuwse delta?
In order to find answers to this question, C. maenas has been separately placed in aquariums under present groundwater conditions. Each aquaria was filled with 30 R. philippinarum shells and one C. maenas. The C. maenas used in the experiment where starved for 48 hours prior of the experiment. When the C. maenas where placed in one of the aquaria they could predate 24 hours on those shellfish. Their where three experiment conducted. Different parameters for those experiments where used, those parameters where crab size, shellfish size and water temperature.
The observations in the first experiment showed no statistical differences between the preferred shellfish size by the C. maenas, the tested shell sizes where ranging between 3 and 12mm. Although there is no particular size preference by the C. maenas in this size range.
The second experiment showed differences in consumption rates between the different sizes of C. maenas used in the experiment. It seems that C. maenas with a carapace width bigger than 50mm consume higher amount of shellfish than C. maenas with a carapace width smaller than 40mm. Expected is that when carapace width increases, energy demand rises and therefore higher predation rates on R. philippinarum occurs.
Finally, the effect of temperature on the consumption rate of the C. maenas was tested the temperatures used where 4℃, 8℃ and 14℃. The results do not show significant differences but they show a higher average consumption at higher temperatures.
Those three experiment led to the conclusion that there is too much difference in in the results to say something about the behaviour of the C. maenas when predating on R. philippinarum, the only proven fact is that bigger C. maenas requires more energy and they’re for consume higher biomass of R. philippinarum. It is recommended that farmers should protect their R. decussatus spat against the C. maenas since it is shown in those experiment that C. maenas with a carapace width lower than 40mm are able to crush R. decussatus spat up to 12mm.